Ensured nonexistant public files fallback to 404

- If we register the serve public file middleware for a file that doesn't exist, this will currently throw an ENOENT error
- Instead, we want to fall back to a standard 404 so that this behaves normally
- This will be useful for the card asset service, where the cards.min.css and cards.min.js files may or may not exist

core/frontend/web/middleware/serve-public-file.js

@@ -7,7 +7,8 @@ const urlUtils = require('../../../shared/url-utils');
 const tpl = require('@tryghost/tpl');
 
 const messages = {
-    imageNotFound: 'Image not found'
+    imageNotFound: 'Image not found',
+    fileNotFound: 'File not found'
 };
 
 function createPublicFileMiddleware(file, type, maxAge) {
@@ -43,6 +44,14 @@ function createPublicFileMiddleware(file, type, maxAge) {
         // modify text files before caching+serving to ensure URL placeholders are transformed
         fs.readFile(filePath, (err, buf) => {
             if (err) {
+                // Downgrade to a simple 404 if the file didn't exist
+                if (err.code === 'ENOENT') {
+                    err = new errors.NotFoundError({
+                        message: tpl(messages.fileNotFound),
+                        code: 'PUBLIC_FILE_NOT_FOUND',
+                        property: err.path
+                    });
+                }
                 return next(err);
             }
 

test/unit/frontend/web/middleware/serve-public-file.test.js

@@ -101,4 +101,25 @@ describe('servePublicFile', function () {
 
         res.end.calledWith('User-agent: http://127.0.0.1:2369').should.be.true();
     });
+
+    it('should 404 for ENOENT on general files', function () {
+        const middleware = servePublicFile('robots.txt', 'text/plain', 3600);
+        req.path = '/robots.txt';
+
+        sinon.stub(fs, 'readFile').callsFake(function (file, cb) {
+            const err = new Error();
+            err.code = 'ENOENT';
+            cb(err, null);
+        });
+
+        res = {
+            writeHead: sinon.spy(),
+            end: sinon.spy()
+        };
+
+        middleware(req, res, next);
+
+        next.called.should.be.true();
+        next.calledWith(sinon.match({errorType: 'NotFoundError', code: 'PUBLIC_FILE_NOT_FOUND'})).should.be.true();
+    });
 });